The present invention relates to a method and a device of importance for shells fired from rifled or smooth-bore gun barrels, which shells during at least some phase of their trajectory are fin-stabilized by fins that deploy from the body of the shell, and which fins during the initial firing phase until the point in time when their stabilizing function is actuated are hinged down against the casing of the shell and are protected by a protector or equivalent this is ejectable when the fins are to be deployed. Furthermore, the present invention is a method and a device that enables the driving band on such shells to be located at its most advantageous position without negatively affecting the shell during the firing phase with undeployed fins.
Artillery shells are usually spin-stabilized through their trajectory until impact with the target or self-detonation or, if the task is to disperse a cargo of, for example, bomblets, until the point in the trajectory at which this is implemented. There are also, however, numerous types of special shells that are spin-stabilized during a greater or lesser part of their trajectory. Shells which are spin-stabilized during a greater or lesser part of their trajectory can either be fired from the barrel with full spin and have the rate of spin decelerated in conjunction with fin deployment, or they can be fired from a rifled or smooth-bore barrel imparting little or no spinxe2x80x94in a rifled barrel by means of a slipping driving band, for example.
There may be several reasons for making an artillery shell fin-stabilized instead of in the simplest and most usual manner letting it be spin-stabilized, but in the case of trajectory correctable munitions or terminally guided shells whose trajectories can be corrected by means of command activated thruster rockets, deployable deceleration devices, guidance devices or equivalent, it is almost an absolute requirement. It is namely much more difficult to correct the trajectory of a fully spin-stabilized body than to carry out an equivalent correction for a fin-stabilized one. As fin-stabilized shells usually have a significantly greater air resistance than corresponding spin-stabilized shells it is usually appropriate, as in the manner indicated above, to allow a shell to begin its trajectory as spin-stabilized and not to go over to fin-stabilizing until the shell approaches its target.
A number of different design principles already exist for using deployable fins for fin-stabilized projectiles. If the projectile in question during the firing phase as well as during a greater or lesser part of its trajectory is spin-stabilized, the same fins may also be initially utilized to retard the rate of spin of the projectile sufficiently to enable these fins to fin-stabilize the shell in the desired manner.
In the purely theoretical type of retractable fins each fin is initially retracted radially in the projectile body or, perhaps more usually, retracted in a dedicated slot or compartment in the projectile body. For the actual deployment function in which the fins flip up or spring up radially there are usually springs incorporated for this purpose. The major disadvantages with this type of fin is that they occupy too much space in the projectile body, and that it is difficult to provide them with sufficient surface area.
A type of fin that occupies significantly less space is the type which initially, i.e. prior to deployment, is retracted snugly curved against and around the projectile body and which, after they are exposed by the ejection of a dedicated protector or the opening of a special retaining device or suchlike, flip up primarily as the result of centrifugal forces. (If the shell is of a general type that is fitted with a slipping driving band and thus has little or no direct inherent spin it will be primarily air resistance forces that power fin deployment.) Fins of this type are usually mounted in the projectile so that at deployment they also rotate around a retaining pin located parallel to the longitudinal axis of the projectile after which they are locked in deployed mode. An example of this type of fin, which in its basic form means that the fin retains its convex shape even after deployment, is Swedish patent no. 339646 in which each fin can be comprised of a piece of sheet metal bent convex around its own pivot and deployment pin. With this type of fin the surface area of the fin usually poses no problem, but on the other hand it is essential to protect the fins while in retracted mode from the gas pressure in the barrel during firing of the projectile. If the propellant gas pressure in the barrel during firing penetrates under the fins the force acting on the fins will be so great that they will deploy too early and too rapidly, resulting in their destruction when exiting the muzzle. In the case of a gas-tight but insufficiently strong protector, the protector would be deformed to such an extent that it would be impossible to eject thus disenabling fin deployment. On the other hand, if the fin protector was made sufficiently gas-tight and stable so as to protect the fins completely it would be far too heavy, costly, and occupy too much space.
In both the Swedish patents 7908002-4 and 8200312-0 two very similar designs are described for base-bleed shells that are initially spin-stabilized, both incorporating fins of the type indicated above but with a somewhat different detail design wherein both are designed to deploy in conjunction with the ejection of the base-bleed unit, and thus subsequently assume a stabilizing function. In both these cases the gas pressure problem during firing has been avoided by locating the fins prior to deployment forward of the driving band, i.e. away from the section of the shell that is subjected to maximum gas pressure. It is, however, not always possible to choose this apparently simple solution to the problem since in reality it is often other criteria that determine where the driving band shall be located along the length of the shell (projectile). As the shell is subjected to its greatest load at the cross-section through the driving band it is usually also necessary to ensure that the shell is extremely resistant to deformation at this point, and it is thus often this requirement that finally determines the location of the driving band.
The purpose of the present invention is that for such shells that are fin-stabilizedxe2x80x94at least during part of their trajectoryxe2x80x94to offer a method and a device to enable the driving band to be located at the optimal position on the shell with regard to general functionality and design without negatively affecting the retracted fins of the shell during the firing phase, which fins are assumed to be convexedly wrapped around the outer periphery of the projectile body and are initially covered by a fin protector. Previously it was standard practice to allow such fins to have a convex shape when retracted around the projectile body to retain their convex form even after deployment. But now that it is possible to obtain material with a sufficiently high degree of elasticity and inherent springback it is possible to manufacture fins which can remain curved against the projectile body in the way indicated above for years, and which still resume an essentially flat shape as soon as they are released/deployed. It is this type of fin to which the present invention primarily relates since it provides certain aerodynamic and other advantages.
As a rule the fins of fin-stabilized projectiles are angled a few degrees relative to the longitudinal axis of the projectile to impart an inherent low rate of spin of the fin-stabilized projectile. Such a slight angling of the fins may also be incorporated in the above indicated type where the fins are retracted again the projectile body, and when deployed whose virtually flat form is achieved by the elasticity and good inherent springback of the material. This slight angling of the fins can also be used to provide deployment force to the fins in the case of projectiles fired with low or no spin at all, such as when fired from a smooth-bore barrel. An equivalent angling of the fins can also be achieved by slightly angling the pivot pin of each fin relative to the longitudinal axis of the projectile.
A special advantage of this method and device as described in the present invention is that it only requires the fins when retracted to be surrounded by an ejectable fin protector of limited material thickness and mass. The basic idea behind the present invention is that the internal volume of the fin protector that initially surrounds the fins that are retracted snugly curved around the outer periphery of the shell, excluding the space occupied by the fins, is completely filled with some sort of appropriate inert, non-combustible, non-glutinous substance with low compressibility and very low inherent strength. For example, there are certain bi-component silicones, including some sold under the name SEALGAARD, that meet these requirements. Another conceivable substance would be a suitable fluidxe2x80x94in the first instance a thixotropic fluid since this would preclude any risk of leakage during depot storage. Another suitable fluid would be water.
With even a relatively lightly dimensioned fin protector filled in this way with a virtually non-compressible substance that completely surrounds the retracted fins, the fins are not susceptible to any kind of damage. Naturally, the fin protector must be fully sealed but it is perfectly adequate for it to be made of small gauge (i.e. limited wall thickness) material as it will withstand extremely high external pressure without suffering deformation that would prevent ejection when the time comes and without any deformation of the enclosed fins. The method indicated above for filling the fin protector also prevents the ingress of propellant gases under the fins which would lead to an excessively rapid opening/deployment of the fins. This means in turn that, as previously mentioned, the driving band of the shell can be located at the optimal position irrespective of whether or not the fin protector and the retracted fins inside are located on the section of the shell that is most affected by the propellant gases. According to the present invention deployment of the fins requires only ejection of the fin protector in question after which, depending on the design of the shell and how it is fired, various combinations of the force with which the fins spring out from their retracted mode, centrifugal forces, and air resistance jointly force the fins outwards to their deployed mode while simultaneously slinging the low inherent strength protective substance from the shell body and fins. The method of using an ejectable fin protector to actuate fin deployment also has the advantage that the same function can also be used to remove, for example, a burnt-out or otherwise no longer desirable base-bleed unit.